The described invention relates in general to welding and joining systems and processes, and more specifically to improved processes and techniques for using thermite welding in various applications.
Exothermic welding, also known as exothermic bonding and thermite welding (TW), is a welding process that employs molten metal for permanently joining metallic materials to one another. The process uses an exothermic reaction of a thermite composition to heat metal and typically requires no external source of heat or current. The chemical reaction that produces the heat is typically an aluminothermic reaction that occurs between aluminum powder and a metal oxide. Despite the overall utility of this form of welding, lapping and lack of fusion are very significant problems with regard to thermite type exothermic welding. Die fit improvements have reduced these problems somewhat but these issues still account for more than 50% of the fatigue failures that occur in thermite-type weld joints in railway applications. Improvement has also been achieved through correcting fit-up issues associated with dies currently in use; however, the problem still exists in many cases. In one example, die fit improvements have focused on preventing liquid iron from simultaneously coming into contact with open atmosphere and cold base material. Another current approach involves adding more molten material to improve the wetting of the filler metal onto the rail surface. This approach leads to an increased heat affected zone size, which is an undesirable outcome.
Exothermic welding is often used in railway (e.g., locomotive) applications, such as for example, welding signal bonds to railroad tracks or joining rail sections to one another. Conventional thermite welding used for railway applications utilizes a mold that wraps around a butt-joint groove that encompasses two rail ends. Considerable effort is spent in mating die fit up to eliminate atmospheric contamination to reduce oxidation of the liquid within the die set. Liquid iron generated by a thermite reaction is then poured into the die set from the top of the mold to fill up the groove, thereby creating a continuous weld between the two rail ends. The most significant area of failure of thermite welds created for railway applications is again due to lapping and lack of fusion presumably due to inadequate die fit up. Lapping occurs when the molten steel flows onto the parent rail material yet doesn't bond or form a contiguous weld joint. This is often identified as a die fit up issue that allows atmospheric contamination to occur and that may create a lack of wetting at the transition from the molten metal filling the die mold cavity and the parent material. As previously described, this lack of wetting is currently overcome by adding excess heat and by adding liquid iron material to increase the temperature in the weld zone. This approach yields some improvements, but still results in an exacerbated heat affected zone softening.
Increasing axle loads and tonnages experienced by North American railroads have increased the demands placed upon all track components, including thermite welds. Thermite rail welds have historically been a weak link in continuous welded rail due to their cast microstructure. The thermite welding method is used worldwide as a field welding method for the final stage in rail installation. Flash-butt welding is mainly used in plant welding, while thermite welding is mainly used in field welding. Thus, there is an ongoing need for improved thermite welding processes for use in various railway applications.